U.S. Pat. No. 3,860,199 discloses in cols. 9 and 10 an electromagnetic roll angle calibrator for a spinning projectile using coils located on the projectile and on its launch platform, respectively. The reference differs from the present invention in that:
1. The reference measures two voltages on the waveform produced by the coil located on the launch platform, one voltage at a peak of the waveform and the other corresponding to the vertical direction. A signal proportional to the ratio of these two voltages is then sent to the spinning projectile, which requires an on-board computer to apply this voltage ratio against the waveform measured by the spinning coil on board the projectile. The present invention measures a phase angle, not a ratio of two voltages; and the corrections can be made on the ground, which is much simpler than introducing a computer into the projectile to make the corrections.
2. The reference apparatus makes no attempt to equalize the rotation rates of the launch platform's coil and the projectile's coil as in the present invention.
3. The reference does not disclose a hold-fire indicator as in the present invention.
4. The reference's projectile's coil 37 is offset from the spin axis of the projectile.
Secondary references are U.S. Pat. Nos. 3,397,358; 3,414,215; 4,109,199; 4,166,406; and 4,328,938.